During the past two decades, many manufacturers of agricultural and industrial equipment have offered enclosed operator cabs for their vehicles. These cabs provide shelter to the operators from the surrounding environment and also muffle the noise created by the vehicle. The industry has readily accepted such enclosed cabs but has voiced concern about the absence of an emergency exit, because in some countries it is a legal requirement. In addition, there is a desire to provide a ventilation passage in the roof of the cab while at the same time sheltering the operator from inclement weather, such as rain. Two prior art patents which provide an emergency exit through the roof of a cab are U.S. Pat. No. 4,099,766 issued to Lorenz in 1978 and U.S. Pat. No. 4,119,340 issued to Wolfe in 1978. The Lorenz patent discloses a roof panel which is completely removable from the cab in the open position while the Wolfe patent discloses a swingable window unit which is pivotable about one side surface and is capable of being positioned either fully open or completely closed. Neither patent teaches an adjustable roof hatch that can be retained in various positions relative to the roof of the vehicle. A third patent, U.S. Pat. No. 3,622,160, issued to Schlapp in 1972 describes a sliding roof cover for a motor vehicle. The sliding roof cover is mounted on a pair of guide rails and is capable of forward and rearward travel relative to the roof of the vehicle. None of these designs fully satisfies the present needs of the industry.